1. Field of the Invention
This invention pertains generally to heat pipe wicks, and more specifically to high performance heat pipe wick structures including those comprising wick materials of 90% or greater porosity.
2. Description of the Related Art
Heat pipes are used in a variety of applications requiring heat transfer mechanisms for transport of thermal energy from one location to another. Heat pipes accomplish energy transfer through vaporizing a liquid in a closed system near a heat source and recondensing the liquid at a different location. Typically, heat pipes include a wick structure that wets with the working fluid to distribute it across a large surface area evaporator thereby facilitating vaporization.
High wick permeability offers low fluid resistance and allows the wick to recharge as vapor evolves off the wick. The result is that, with greater permeability (which often is associated with high porosity), more liquid is supplied during application of heat, and therefore, more heat can be transferred without wick dryout. An open structure made of very little material, however, is structurally weak. Consequently, wicks with high porosity and excellent fluid flow characteristics tend to lack durability in the absence of other mechanical support.
Typical wick structures deployed, for example, in dish Stirling solar engines, use either powdered metallurgy or woven wire screens to provide the wicking pores. Although these have limited porosity and permeability, they usually have good structural and durability properties due to the large amount of internal structure they exhibit. Durability is required, for example, in Stirling engines, where the liquid to be evaporated (for example, molten sodium) is carried upward from a reservoir through a wick by capillary movement. As the wick becomes loaded, the weight of the liquid in the wick exerts pressure that, without sufficient support to counteract the load, can cause the wick to deform or collapse. For low porosity wicks, the mechanical load can be supported by the internal wick structure, itself. However, for higher porosity wicks, such as those comprising randomly-laid fine metal fibers, collapsing (or inflating, where bubbles disrupt wick integrity) pose a serious challenge, especially where wick lifetimes of tens of thousands of hours are desired.
A need remains, therefore, for heat pipe wick structures that exhibit high porosity and permeability but are durable and can withstand, over the long term, mechanical loads and stresses encountered during normal operation.